A conventional radio channel assignment method is explained below.
The conventional radio channel assignment method includes, for example, an autonomous distributed dynamic channel assignment scheme in which each of two or more radio base stations perform channel selection/assignment autonomously without exchanging control information between other radio base stations (see Patent Literature 1).
When there is a call, the conventional autonomous distributed dynamic channel assignment scheme allows a base station or a mobile station to select all of radio channels which are used by a system. Each of base stations cyclically measures received levels of interference at radio channels, channel-by-channel, and enters radio channels, at which the received level of the interference is an allowable value or less in the system, in an idle channel table on a memory of the system in order of detection of the radio channels and by the fixed number thereof.
Furthermore, upon switching of radio channels or upon finishing of communication, each of the base stations sets the order of received-level measurement of interference at a radio channel to the highest order, the radio channel at which no interference is affected during communication and which is released thereafter. Each of the base stations also sets the order of received-level measurement of interference at a radio channel to the lowest order, the radio channel at which interference is affected during communication and which is released thereafter.
When there is a call, the base station refers to the memory of the system and determines whether the communication link quality satisfies the allowable value of the system in the order from the radio channel that is entered in the idle channel table as the highest order. The base station then selects the radio channel initially determined that the quality is satisfied, and uses it for communication.
Patent Literature 1: Japanese Patent No. 3244153, Channel assignment method.
However, in a radio communication system in which the channel assignment method described in the above patent literature is used, the base station refers to the received levels of interference cyclically measured. Therefore, this system does not recognize the received level of interference at the transmission timing. Because of this, if a re-request of radio channel assignment or a radio channel switching request is received from a mobile station, and if switching is instructed by a base station, instead of assigning a carrier that is appropriate at the transmission timing, a carrier near a carrier that is last assigned is generally assigned.
Furthermore, in the radio communication system in which the conventional channel assignment method is used, in neighboring base stations, for example, there is a case where carriers, at which a received level measured value of interference satisfies an allowable value or less in the system, may overlap, and a timing of assigning a radio channel may be close to another timing. In this case, the assigned carriers overlap, which increases the interference between base stations.
The present invention has been achieved to solve at least the conventional problems. It is an object of the present invention to provide a radio channel assignment method capable of assigning an appropriate carrier at a transmission timing, as a radio channel, (capable of reducing probability of assigning a carrier near a carrier that is last assigned), if a re-request of radio channel assignment or a switching request thereof is received from a mobile station or if switching is instructed by a base station.
It is another object of the present invention to provide a radio channel assignment method capable of reducing interference between base stations through distribution of carriers, which can be assigned, over wide frequency bands by a neighboring base station.